The design criterion of mobile air conditioning systems requires that the air conditioning systems be designed to operate as efficiently as possible over a wide variety of environmental and operating conditions in a manner so as not detrimentally impact the capability of the equipment to properly perform, particularly the compressor or its clutch, as environmental and operation conditions change. One of the demands on operation of the compressor or its clutch is created by mobile air conditioning systems evaporator de-icing control. Evaporator icing is caused when the evaporator temperature is dropped to a level, that when air is blown through the evaporator, the humidity in the air causes ice crystals to form on the fins and tubing within the evaporator. If this continues for an extended period of time, the ice so formed will impede the flow of air through the evaporator, reducing the cooling capacity of the air conditioning system, and in the extreme, if sufficiently entirely iced over, essentially cutting off most of the air flow, thereby effectively incapacitating the air conditioning system.
The mobile air conditioning systems of the prior art, such as,those included in automobiles, trucks, busses, etc., presently include a temperature or pressure sensor at the output of the evaporator to detect when the evaporator is operating at a condition that is likely to cause icing. When the icing condition is sensed, the sensor reduces the compressor refrigerant flow capacity by operating a clutch relay, or switch, to deactivate the compressor clutch and thereby stop refrigerant flow until the sensor detects the selected non-icing condition. Such mobile air conditioning systems are usually driven by the vehicle engine through an electrically operated clutch. The clutch is initially activated when the air conditioning system is turned on, and then turned on and off by the vehicle air conditioning system temperature control system to maintain the vehicle at the desired temperature. In the later vehicle models, the evaporator icing sensor is connected to a vehicle computer that controls the activation and deactivation of the clutch.
It has been found that the higher the humidity of the atmosphere, more likely that icing can occur in the evaporator. Hence, mobile air conditioning systems are more susceptible to icing in the more humid, locations, which can be considered as the "worse case" for design purposes. As in most mechanical devices, mobile air conditioning system clutches have design limits as to the number of times the clutch can be cycled on and off over a period of time with out causing deterioration or break down. In the case of automobile air conditioning systems, it is recommended that the compressor clutch should cycle on and off no more than six times a minute. Hence, it is important that the design of mobile air conditioning systems operates efficiently over a wide range of environmental and operating conditions, without evaporator icing, and without causing excessive cycling of the compressor clutch.
The present approach of the prior art to maintain a balance between evaporator deicing control and compressor cycle rate is to include a large differential in the icing detector sensor between selected non-icing and icing conditions. The large sensor differential was selected was that needed for worse case conditions (i.e. expected worse case humidity, temperature, etc. conditions). The sensor was set so that the selected large differential, along with the response time of the air conditioning system, assures that deicing system clutch cycling demands are within recommended cycling design limits at worse case conditions. For example, in some models of automobiles the icing condition is selected at 25 psi., while the non-icing condition is selected at 45 psi. (system reset condition), a significant differential of 20 psi. Although this arrangement was found to be a satisfactory to prevent evaporator icing in mobile air conditioning systems for worse case conditions, this large differential penalizes the performance of the air conditioning system in other than worse case conditions by keeping the compressor in the off condition over a wider range of the temperatures than needed. For example, if the compressor is shut off at the icing condition of 25 psi., the entire air conditioning system remains shut off until the preset differential high limit of 45 psi. is reached, a significant dead time required for the worst case operation. Since such large differential was selected for the worse case condition, the same large differential exists for all other operating conditions, despite the fact that the large differential is not needed and thereby unnecessarily detrimentally impacts the performance of the system under the large majority of operating conditions by keeping the system shut down longer than needed. It would therefor be advantageous if the differential detection range between the icing and non-icing conditions could be reduced to improve the overall performance of the air conditioning system while still not exceeding the recommended clutch cycling rate.
Further, the use of a pressure sensing compressor clutch deactivating system of the prior art was found to be undesirable in mobile air conditioning systems using a temperature sensitive refrigerant control valve (that controls the refrigerant flow through the system). Such temperature sensitive control valves are connected in the output line of the evaporator to detect the evaporator output temperature and control the system refrigerant flow as a function of evaporator output superheat. The temperature sensing mechanism of the control valve is inherently slow in its reaction both in its refrigerant flow restricting and increasing flow modes. With a pressure sensing evaporator icing control included in such system, it was found that when the control valve cuts back refrigerant flow and an icing condition is simultaneously sensed, the combination of the response times of the operation of the valve and the icing control system interact to cause the compressor clutch to rapidly cycle off and on several times before the system is stabilized. This clutch cycling not only is detrimental to the life of the clutch but was found to be annoying to the vehicle operator. It would therefor be advantageous to be able to include an evaporator deicing control system in a mobile air conditioning systems using a temperature controlled control valves that would be operable in such systems and still avoid the repetitive cycling of the compressor clutch due to valve and icing system interaction.
It is therefor an object of this invention to provide a new and improved compressor flow control arrangement for controlling compressor or clutch cycling in mobile air conditioning systems that provides for the use of the evaporator de-icing control arrangement that has a significantly reduced range between icing and non-icing determinations to thereby improve the overall system performance while maintaining compressor clutch cycling within design limits.